The present invention relates to an image position adjusting device in an offset printing machine capable of printing on both sides of a sheet of paper.
In the field of offset printing, the demand for paper printing plates has recently increased following the development of paper plate manufacturing machines because the manufacturing cost per plate is low and the plate can be automatically mounted on the plate cylinder and removed therefrom. However, these plates suffer from a disadvantage that often the position of an image on the plate is shifted from that of the original during the plate manufacturing process. The amount of shift depends partially on the accuracy of the plate manufacturing machine itself. However, the amount of shift is also dependent upon the operator's skill in positioning the image part at a predetermined position on the plate. This is a considerably difficult operation in practice as the image part is liable to shift for every plate.
Because of the shift of the image part, during a printing operation it is necessary to readjust the image position to correctly determine the printing image position with respect to supplied sheets whenever the printing plate is replaced.
In an offset single-side printing machine, the plate cylinder, the rubber cylinder and the impression cylinder are coupled and driven by separate gears provided therefor while in an offset printing machine capable of printing on both sides, the plate cylinder and the rubber cylinder which are paired for the front side of a printing sheet and the plate cylinder and the rubber cylinder which are paired for the rear side are coupled and driven by gears provided therefor in such a manner that the cylinders are driven at the same circumferential speed and are maintained in a predetermined circumferential relation, that is, with a constant angular relation. With this arrangement, the image position can be adjusted by changing the circumferential coincidence relation between the cylinders.
A conventional image position adjusting device for a single-side printing machine is shown in FIGS. 1 and 2. A gear a' is detachably secured to a rubber cylinder a with a retaining screw b and is maintained engaged with a gear c' secured to an impression cylinder c at all times and establishing a circumferential coincidence relation between the rubber cylinder a and the impression cylinder c. To adjust the image position, the retaining screw b is loosened by turning a knob d' provided at one end of a box wrench d to release the gear a' from the rubber cylinder a. Thereafter, a gear g which is fixedly mounted on a shaft f supported by frames e and which is engaged with the gear a' is turned with a handle h provided at one end of the shaft f to thereby turn the impression cylinder c through the gears a' and c' as a result of which the circumferential coincidence relation between the rubber cylinder a and the impression cylinder c is changed to achieve the image position adjustment. Thereafter, the retaining screw b is tightened with the box wrench d to fasten the gear a' to the rubber cylinder a.
In the above-described conventional device, after the screw b has been loosened, the impression cylinder c is manually turned to change the circumferential coincidence relation between the cylinder c and the rubber cylinder a to achieve the image position adjustment. In reality, the adjustment is achieved by trial and error. Accordingly, it takes a long time for even a skilled person to accomplish the image position adjustment. In addition, when the screw b is tightened after the image position adjustment, the image position may again be shifted. Thus, the image position adjustment with the conventional device is low in accuracy and accordingly often it is necessary to repeat the adjustment several times.
Moreover, the image position adjustment is carried out with the printing machine stopped. Therefore, as the period that the printing machine is stopped increases, the plate surface becomes more and more dry as a result of which ink sticks to parts of the plate other than the image part. It is necessary to remove the excess dried ink by idling the machine although the printing operation could otherwise be started because the image position adjustment has been achieved. In the case of a paper printing plate, if ink sticks to parts of the paper plate other than the image part, it is impossible to use the paper plate again.
If the above-described conventional device is employed for the printing machine capable of printing on both sides, the accuracy becomes worse and the time required for image position adjustment is increased.
An object of the invention is thus to accomplish the front side image position adjustment and the rear side image position adjustment readily and quickly without requiring a high amount of operator skill.